THE MAIN LYTIC FACTOR OF TRYPANOSOMA-BRUCEI-BRUCEI IN NORMAL HUMAN SERUM IS NOT HIGH-DENSITY-LIPOPROTEIN

Natural immunity of humans to the cattle pathogen Trypanosoma brucei b rucei has been attributed to the presence in normal human serum (NHS) of lytic factors for the parasites. We and others have shown that NHS contains two trypanolytic factors (herein termed TLF1 and TLF2) that c an be separated by gel filtration. TLF1 copurifies with a subclass of high density lipoprotein (HDL), whereas TLF2 has a much higher molecul ar weight and does not appear to be a lipoprotein. We find that the tr ypanolytic activity of purified TLF1 is totally inhibited by exogenous haptoglobin (Hp) at concentrations (0.1 mg/ml) lower than those prese nt in NHS (0.2-2 mg/ml). In contrast, exogenous Hp (up to 2.5 mg/ml) h as no effect on the lytic activity of either NHS or isolated TLF2. Hp- depleted sera from patients with intravascular hemolysis is severalfol d more trypanolytic than NHS. These sera contain only TLF1, and their lyric activity is totally abolished upon the addition of Hp (0.1 mg/ml ). When NHS containing different Hp allotypes is fractionated by gel f iltration, TLF1 activity is either revealed or remains masked, dependi ng on whether it coelutes with Hp. Masked TLF1 activity in the column fractions is revealed if Hp is removed by density gradient ultracentri fugation. We conclude that endogenous Hp inhibits TLF1 activity, and t hat TLF2 is the main trypanolytic factor in NHS.